BOSTON — The Coast Guard is investigating the cause of a tug collision that occurred about three miles south of Kennebunk, Maine, early Thursday, and monitoring the site where one of the stricken vessels sank.

One of the two crewmen aboard the 40-foot tugboat Helen Louise alerted Coast Guard watchstanders Wednesday that their vessel collided with the 80-foot tugboat Capt Mackintire while they had it in tow. The Capt Mackintire had no crew aboard.

A response boat crew from Coast Guard Station Portsmouth Harbor, in New Hampshire, and the 87-foot Coast Guard Cutter Reef Shark, homeported in Boston, deployed to assist.

Once on scene, the Coast Guard Station Portsmouth Harbor boat crew confirmed there were no injuries to the two-man tug crew. The Coast Guard then escorted the tug Helen Louise, with the two people aboard, into Portsmouth, New Hampshire, where it safely moored.

The Coast Guard Cutter Reef Shark established a tow with the tugboat Capt Mackintire, with intentions to bring the boat into Portland, Maine.

While in transit to Portland early Thursday, the Capt Mackintire began taking on water, forcing the crew to cut the towline where it sunk in about 158 feet of water.

The Coast Guard is working with federal, state, and local authorities to evaluate pollution potential and respond to reports of sheening in the area where the tug sank.